This invention relates to electric power steering systems.
Electric power steering (EPS) systems have been the subject of development by auto manufacturers and suppliers for over a decade because of fuel economy and ease-of-control advantages compared with the traditional hydraulic power steering (HPS). Commercialization of EPS systems, however, has been slow and is presently limited to small and midget-class cars due to cost and performance challenges. Among the most challenging technical issues is the pulsating feel at the hand wheel (also referred to as torque ripple) and the audible noise associated with the type of high performance electric drives needed to meet the steering requirements.
Existing EPS systems employ various methods of controlling a sinusoidal brushless motor to assist steering. The power to the sine motor is provided by a by a electric power converter. The power converter converts the input voltage from a constant voltage source to desired voltage by controlling the width (also known as duty cycle) of the voltage pulses during a constant switching period. Also the sinusoidal shape of the line voltage and therefore the current is achieved by pulse width modulating the input voltage with respect to the motor position. Two existing control methods are the 50 percent duty modulation (or dual edge modulation) method and the phase grounding method. In 50% modulation technique all three phases are pulse width modulated resulting into a positive and negative voltage is being applied across the phase during a switching period so the at the average voltage across the phase is equal the desired voltage at the operating point. In phase to grounding method technique each phase of the motor is completely grounded for 120 electrical degrees and the other two phases are pulse width modulated resulting into a unipolar voltage across the phases the average of which is equal to the desired voltage across the phase at that operating point. Each method has advantages and disadvantages, but the 50 percent duty modulation method has been preferred due to its superior torque ripple xe2x80x9con-centerxe2x80x9d (i.e., when the vehicle hand wheel is close to its center position). In phase to grounding technique at very small torque levels which happens when the steering wheel is near the center the desired width of the voltage pulses is very small. This also appears near the zero crossing of the sine wave. Due to the dead time (where both switched in a phase leg are off) between the switches and the nonlinearities and propagation delays in the switching devices, no voltage appears across the phase until the desired voltage and therefore the duty cycle reaches a thrush hold and then the voltage starts to increase in a nonlinear fashion. This nonlinear change in the voltage creates an undesired on-center feel in the electric power steering systems. Thus, there is a need for improving the on-center torque ripple in phase grounding controlled EPS systems.
An exemplary embodiment of the invention is a method of controlling a motor in an electric power steering system. The method includes modifying the desired duty cycle to compensate for the nonlinearities caused by the switching devices that appear in the voltage and therefore in the current and torque of the motor. This is achieved by obtaining a motor control signal having an initial duty cycle and linearizing the initial duty cycle to obtain a secondary duty cycle. An offset is added to the secondary duty cycle to obtain an adjusted duty cycle. The motor is controlled using the adjusted duty cycle.
An alternative embodiment is a torque linearization device for linearizing torque produced by a motor in response to a motor control signal having an initial duty cycle.
The torque linearization device includes a comparator for receiving the initial duty cycle and comparing the initial duty cycle to a threshold (below which the linearization is needed). If the duty cycle is below the threshold a table converts a linearization table converts the initial duty cycle to a linearized duty cycle. A selector receives the initial duty cycle and the linearized duty cycle and selects one of the initial duty cycle or the linearized duty cycle as a secondary duty cycle. An offset is then added to the secondary duty cycle.